1. Field of the Invention
This invention relates to surface controlled subsurface safety valves and seals.
2. History of the Prior Art
It is well known to complete oil and gas producing wells with flow control systems which include subsurface safety valves controllable from the surface to shut off fluid flow in the tubing string of a well. Generally, such valves are controlled in response to a control fluid pressure conducted to the valve from a location at the surface end of the well so that the well may be selectively shut in responsive to predetermined conditions which may include rupture of a flow line, fire, and the like. The safety valves may be of either the wireline or tubing retrievable type. The wireline type subsurface safety valve is inserted into and removed from a landing nipple in the tubing string of a well by use of standard wireline equipment and techniques. The tubing retrievable type subsurface safety valve includes a housing which is connected into and becomes an integral part of the well tubing string so that the valve is installed and removed with the tubing string. Both types of subsurface safety valves include a pressure responsive operator piston which often must function under adverse conditions such as the presence of hydrogen sulfide and high temperatures and pressures. Elastomers often used in downhole seal applications frequently will not operate satisfactorily under such well conditions. A further factor affecting the integrity of seals used in subsurface safety valves is that the seals must be effective under both static and dynamic conditions. The operating pistons in the safety valves must move to open and close the valves so that a seal must be achieved between adjacent surfaces, one moving relative to the other.
Metal-to-metal seals have been used in downhole applications to overcome environmental problems such as corrosive fluids and high temeratures and pressures. For example, in U.S. Pat. No. 4,429,620, a metal-to-metal seal is shown to seal under static conditions. While the patented seal is supported around a moving piston, a seal is effective only at the end of each stroke of the piston responsive to mechanical compressive forces which expand the seal into contact with an adjacent surface. Thus, the seal is not designed to function in response to fluid pressure and under dynamic conditions when the piston is moving relative to the cylinder wall. Similarly, U.S. Pat. No. 4,346,919 shows a metal-to-metal static seal. U.S. Pat. No. 4,452,310 shows a dynamic metal-to-metal seal which is not, however, structurally adaptable to seal the annular space around the operator piston of a subsurface safety valve as in applicant's device. Additionally, the seal assembly of U.S. Pat. No. 4,452,310 is more complex and more expensive to manufacture and install than the seal disclosed by applicant.